change dockerfile to build using yarn

Signed-off-by: Alexander Schwartz <alexander.schwartz@gmx.net>

Dockerfile

@@ -32,7 +32,7 @@ RUN echo -n "Node.js version " && node --version \
   && hugo version
 
 RUN bundle \
-    && npm --unsafe-perm ci \
+    && yarn \
     && hugo --gc --minify --destination public
 
 FROM eclipsefdn/nginx:${NGINX_IMAGE_TAG}